This invention relates to a process for the separation of finely divided clay particles from aqueous suspensions thereof and, more particularly, to an electrokinetic process wherein the aqueous suspension is effected simultaneously by electrophoresis and electroosmosis.
Clay such as kaolin, bentonite and the like are widely used as adsorbents, pigments, catalysts, paper fillers and the like. These clays generally occur in geological deposits as mixtures with inert foreign materials, and it is necessary to separate the clay material before the clay is utilized in commercial applications.
In effecting the separation, the crude clay is dispersed in water with the aid of dispersing chemicals such as sodium silicate, sodium hexametaphosphate, sodium tripolyshosphate and tetrasodium pyrophosphate and the clay slurry is blunged, degritted, classified and leached to effect removal of the undesired foreign materials. Following these steps, the clay is filtered to remove the dispersing and leaching chemicals and to produce a solid filter cake containing 50% to 60% clay. This 50% to 60% solids cake is then redispersed and spray dried or otherwise dried in the flocculated state. For various economic reasons it is desirable in the clay industry to ship slurries containing 70% solids. Therefore, it is conventional in the clay art to add approximately 35% to 50% spray dried clay to a 50% to 60% solids redispersed filter cake to yield a 70% solids slurry. The addition of spray dried clay to clay filter cake to prepare a 70% solids slurry for shipment adds significantly to the high cost of the shipped slurry due to the relatively high costs of preparing the spray dried clay. The art, therefore, has been continually seeking to effect methods whereby clay filter cakes containing 70% clay can be directly obtained from clay suspensions without the addition of spray dried clay.
Among the methods which the art has investigated it its attempt to obtain a more concentrated, i.e., 70% solids clay cake, has been the use of electrokinetic phenomenon such as electrophoresis and electroosmosis.
Most solid materials when suspended as fine particles in a liquid acquire an electric charge. By applying a DC (Direct Current) electric field between two electrodes immersed in the particle suspension, the particles are caused to travel toward one of the electrodes and form a deposit thereon. This travelling of solid particles through a liquid due to the application of DC is referred to in the art as electrophoresis. When under the influence of a direct current potential, water or other liquid medium is caused to migrate through a stationary porous diaphragm toward a charged electrode. This phenomenon is referred to in the art as electroosmosis. Both electrophoresis and electroosmosis have been applied to the separation of clays from aqueous suspensions thereof. When applied to aqueous clay suspensions, electrophoresis is generally used to effect the deposition of the suspended clay material on a charged electrode, whereas electroosmosis functions as an aid in consolidating and concentrating the electrophoretically deposited clay by removal of the entrained water from the deposit.
General descriptions of these phenomena may be found at Kirk-Othmer, Encyclopedia of Chemical Technology, V. 5 (1950, pp. 549-551, 606-610, V. 7, (2d Ed. 1965) pp. 841-865, V. 8, (2d Ed. 1965) pp. 23-36, Interscience Encyclopedia, Inc., N.Y., N.Y.; Poole-Doyle, Solid-Liquid Separation, pp. 44-59, 100-188, 471-479, 511-513, 627-634, 698, 749, 750, 782, 835-844, 869, 873, 922, 923, Monlik, "Physical Aspects of Electrofiltration", Environmental Science & Technology, V. 5, No. 9, September 1971, pp. 771-776; Reif, "Electrokinetics", Industrial Research, December 1971; Creighton, Electrochemistry, V. 1, pp. 143-165, John Wiley & Sons, Inc., N.Y., N.Y. 1943; Sennett-Olivier, "Colloidal Dispersions, Electrokinetic Effects, and the Concept of Zeta Potential" Chemistry and Physics of Interfaces, pp. 73-92 American Chemical Society, Washington, D.C., 1965; Daniels-Alberty, Physical Chemistry, pp. 512-516, John Wiley & Sons, Inc., N.Y., N.Y., 1955; Glasstone, Textbook of Physical Chemistry, 2d Ed., pp. 1219-1240, D. Van Nostrand Co., Inc., Princeton, N.J., 1946.
Descriptions of some specific applications may be found in Curtis, C. E., "The Electrical Dewatering of Clay Suspensions", J. Am. Ceram. Soc., 14, 219 (1931); Drever, J. I., "The Separation of Clay Minerals by Continuous Particle Electrophoresis", Am. Mineralogist, 54, 937 (1969); and Miller-Baker, "Electrophoretic-Specific Gravity Separation of Pyrite from Coal", Report 7440, Bureau of Mines, Dept. of Interior, (1970).
In addition, the following patents are representative of the prior arts attempts to employ those phenomena in practical applications: Ser. No. 936,805 (1963); U.S. Pat. Nos. 670,350; 720,186; 894,070; 972,029; 993,888; 1,121,409; 1,133,967; 1,156,715; 1,174,946; 1,233,713; 1,229,203; 1,235,063; 1,266,329; 1,326,106; 2,099,328; 2,236,861; 2,295,476; 2,440,504; 2,448,848; 2,485,335; 2,500,878; 3,396,097; 3,412,002; 3,412,008; 3,455,805; 3,497,439; 3,533,929; 3,556,969; 3,589,991; and 3,616,453.
While the effect of electrophoresis and electroosmosis on clay separation has long been known, they are not commercially successful by the present day art. Attempts by the art to replace the filtration procedures conventionally used to prepare clay filter cakes from clay suspensions with electrophoretic or electroosmotic devices have not been notably successful, particularly from an economic standpoint.